206 USE OF I'HE MICROSCOPE. 



light required ; the planes are about iV of an inch in width. 

 The condenser is set in a frame of brass as shown in perspective 

 in No. 2, or in section in No. 3, and is applied to the brasswork 

 of the ordinary achromatic condenser, the central part, c, being 

 stopped up with black wax or other dark substance. When 

 rays of light pass through the under part of the ring and 

 impinge on the surface, b, they are reflected from that surface 

 and are transmitted through that of a upon any object placed 

 upon the stage, the angle being dependent upon the inclination 

 of the surface, b, so that the rays fall at the same angle and in 

 every azimuth of the circle. 



The above was the form of condenser first recommended by 

 Mr. Shadbolt; he has, however, recently effected a very 

 material improvement in its construction, whereby its illumin- 

 ating powers are greatly increased. In the former instrument 

 all three surfaces were rectilinear; but in the present form 

 the reflecting and emergent surfaces are curves, the outer one 

 being a close approximation to a parabola. This new illumin- 

 ator is represented of the size for a low power in fig. 134 : a is 

 the outer parabolic surface ; b, the brass-setting ; c, the dark 

 spot in the centre of the concavity, d. As thus constructed, 

 this new instrument somewhat resembles a cast in glass of the 

 interior of the illuminator of Mr. Wenham, but is more easy 

 to use. Mr. Wenham's, however, is capable of being employed 

 with most of the object-glasses as high as the \, from having 

 an adjustable stop in the centre ; but Mr. Shadbolt's requires 

 a distinct illuminator for each object-glass. There are some 

 objects that are beautifully seen by this illuminator as well as 

 by that of Mr. Wenham : these, however, wUl be particularly 

 pointed out in the classified lists. 



Mr. Gillett's Condenser. — This very valuable instrument, 

 contrived by Mr. GUlett, is constructed by Mr. Koss; it is 

 represented of the fuU size in fig. 135, and consists of an 

 achromatic combination. A, about equal in power to 5 of an 

 inch, and having an angular aperture of 80", beneath which 

 a conical diaphragm, B, provided with 22 apertures of dif- 

 ferent sizes is fixed, and each aperture in succession becomes 

 the limiting aperture of the illuminating combination, A. 



